Most scanners used in X-ray or in nuclear medicine computerized tomography have a radiation detector mounted on a rotor which rotates about a patient to acquire data. The data is employed in reconstructing a tomographic or planar image of a desired section of the patient.
In X-ray computerized tomography the scanners in present use are designed to operate as either rotate-rotate ("third generation") scanners or "rotate-only" ("fourth generation") scanners. In both cases a gantry including the rotor and stator are provided with a central axial aperture concentric with the axis of rotation of the rotor. The aperture is designed to conveniently receive a normal person in a prone position.
Data from the detector when mounted on the rotor must be transferred to processing equipment that remains stationary. In addition, operating power and control signals have to be supplied to the rotor to control the operation of an X-ray tube, among other things.
In SPECT scanners the gamma ray camera head or detector is mounted on a rotatable portion of a stationary ring to enable the gamma camera head to circle the patient. Thus, data is acquired from many different rotational angles about the patient thereby enabling reconstruction of a tomographic image. Here again, control signals and operating power are required for operation of the camera head and data from the camera head must be transferred from the rotary ring to the computer of the gamma camera system which remains stationary.
Conventionally, the required power and data including control signals are transmitted to and from the rotatable member both in the X-ray scanner and the nuclear medicine scanners via flexible high-voltage cables for the power and shielded cables for the control signals and the data. Cable uptakes or spooling systems have been provided which enable at least one complete rotation of the rotatable member to occur.
More recently, new designs have been used for transferring both data and power to and from the rotatable member. See, for example, U.S. Pat. No. 4,912,735, entitled "Power Apparatus Particularly for CT Scanners", which issued on Mar. 27, 1990, and which is assigned to the Assignee of this invention. That Patent describes unique inductive power transfer methods which enable discarding flexible cables and the spooling systems for the transmission of power in X-ray computerized tomography.
However, until the invention of the copending U.S. application Ser. No. 785,056 filed Oct. 30, 1991 data and control signal communications between the rotating and stationary parts of the system for acquiring SPECT images always used flexible cables and/or cable pulley systems attached to the rotatable portion of the scanning apparatus. The system of the aforementioned Patent Application taught the possibility of SPECT scanners to rotate more than once about the patient.
Data and control signal communication apparatus coupling rotating and stationary portions of gantries, particularly for X-ray CT scanners, are found for example in U.S. Pat. No. 4,796,183 which covers a system that transmits data between a rotor and a stator utilizing a wave guide attached to the rotor.
Another pertinent data and control signal inter-communication system is described in U.S. Pat. No. 4,259,584. There data generated by the detector of a CT scanner is transmitted to stationary processing equipment using a ring of light conducting material bent around the center of rotation of the rotatable member to form a ring. A light source emits light signals that correspond to the data signals. The emitted light signals are transmitted on to the ring of light conducting material. The ring conducts the light signals over its circumference to a coupling location at which a light receiver is located on the stationary part of the scanner.
A light utilizing system for communicating data and control signals between a stator and a rotor is the system disclosed in U.S. Pat. No. 5,134,639 issued Jul. 28, 1992 and which is assigned to the Assignee of this Application. That Application utilizes a hollow tube having a reflective inner surface for transmitting data and control signals on modulated light beams between a rotating member and a stationary member of a computerized tomographic system.
In summary, the pertinent prior art on data and control signal communications between rotor and stator using light as the communicating media shows two different modes for the transmission of data and/or control signals. In one mode light conducting material curved around the center of rotation is used (U.S. Pat. No. 4,259,584). The other mode is that of U.S. Pat. No. 5,134,639 which teaches the use of hollow tubes with reflective inner surfaces with the light being projected axially.
Thus, the prior art transmits control and data signals to and from the rotating part of the gantry in a manner enabling continuously rotating the gantry over many revolutions without having to reverse and return to the zero degree point after each revolution as was required when cables were used for coupling the rotary part of the gantry to the stationary part of the gantry. However, the prior art used for transmitting signals to and from the rotary part of the gantry either features limiting solid light transmitting material, or delicately machined hollow tubes.
The problem of providing optical communication links between the rotor and stator of medical imagine gantries imposes severe restraints. For example, there is a space constraint which limits the transmission medium to a circular ring shaped cavity. The rotor and the stator transmitters and receivers must move within this cavity each describing a circle of rotation without obstructing each other while maintaining continuous communication between stator and rotor. At the same time there should be no cross-talk between transmitter and receivers of the same unit; i.e., stator or rotor.
Since the use of the transmitted data is critical, there is a requirement of extremely low bit error rate; i.e., BER 10.sup.-12. This makes it preferable that the light flux from transmitter to receiver be optimized. Optimization in this context means not only maximum signal-to-noise ratio at the detector, but especially minimization of the dynamic range of the light flux variation.